Tone-control system



March 25, 1941. N. P. cAsE TONE-CONTROL SYSTEM Filed Dec. 16, 1939 no sh.

-nuodsou ummm INVENTOR NELSON P. SE y ATTORNEY Prasad Mar'. 25, 1941 'roNn-coNrnoL srs'rEM Nelson P. Case, Great Neck, N. Y., signor to Hazeltine Corporation, a corporation of Dela- Wale Application nece'mber 1s, 193s, semi No. 309,551

(c1. zso-zo) 8 Claims.

'I'his invention relates to tone-control systems and particularly to selective tone-control systems for multiband modulated-carrier signal receivers.

Multiband receivers, particularly when receiving modulated-carrier signals in the short-wave band of frequencies, are troubled with noise resulting from both natural and man-made electrical disturbances. In such receivers of the prior art it has been customary to provide tone-control systems for attenuating the higher audiofrequency signal components which include a large portion of the noise energy, However, in the operation of such receivers in the short-wave bands there may be present also a considerable amount of noise energy in the low-frequency signal components. It frequently occurs, however, that such receivers are operated in the short.- wave band to receive speech rather than music, in which case a ther narrow frequency band is suflicient to pro e satisfactory intelligibility. It is desirable, therefore, in the receivers adapted to operate in high-frequency bands, to include protranslating attenuating network coupled between the input and output circuits. v The attenuating network includes a plurality of signal-attenuating portions, one of which is of the high-pass type and contains in series a resistor and a condenser having a time constant of not less than 0.0002 second and not greater than 0.0010 second and another of which is of low-pass type and contains in parallel a resistor and a condenser' having a limiting frequency eective to attenuate by at least 6 decibels all signals above a limiting frequency lying between 800 and 1500 cycles, whereby the network effectively attenuates all noise and sound signals at frequencies outside of thefrequency range of 100-500 cycles to 800-1500 cycles, that is, the middle range of frequencies that contribute most to intelligibility. The network also includes means for effectively progressively transferring the second condenser from such other signal-translating portion to the one signal-translating portion, simultaneously and progressively visions for attenuating not only the higher-frequency signal components but also the lower-frequency signal components and to translate and reproduce only the middle range of audio-frequency components most necessary for intelligibility of speech.

It is an object of the presentinvention to provide a tone-control system for a modulated-carrier signal vreceiver for eiectively attenuating noise and sound signals at frequencies outside of the middle range of frequencies that contribute most to intellgibility, u

' It is another object of the invention to provide a tone-control system for a multiband modulated-carrier receiver including unicontrol means for simultaneously' switching the band selector and adjusting the tone control whereby predetermined tone-control characteristics may be selected for each frequency band.

It is a further object of the'invention to provide an improved tone-control system, the time constants of which may be adjusted until the system translates only audio-frequency signals effectively within a predetermined narrow range of frequencies that contribute most to intelligibility.

In accordance with the present invention, an i audio-frequency amplier of a modulated-carrier signal receiver includes a tone-control system comprising an input circuit for supplying audiofrequency signals and incidentally including noise which may extend over the entire audio-frequency range, an output circuit, and a signal- 'to change the time constants of both of the signal-attenuating portions, thereby progressively to change the frequency-band response of the network. Y I

In the preferred embodiment of the invention, the high-frequency attenuating portion of the network is provided witha resistor having an adjustable contact, and the condenser of the lowfrequency attenuating portion of the network is connected to the adjustable contact, the adjustment of the contact being effective progressively to vary the time constants of the network between the values forwhich the network translates the useful audio-frequency components and the values for which the network effectively attenuates all noise and sound signals at frequencies outside of the middle range of -frequencies that drawing, and its scope will be pointed out in the appended claims.

Fig. 1 is a circuit diagram, partly schematic, of a complete multiband superheterodyne receiver embodying the invention; Figs. 2 and 3 are simpliiied circuit diagrams for explaining the operation of the tone-control system; while Fig. 4 illustrates response-frequency characteristics of f the tone-control system of Fig. 1.

Referring to Fig. 1 of the drawing, there is shown, partially schematically, a superheterodyne receiver including an antenna-ground circuit IIi, I0 to which is coupled a radio-frequency amplifier II having a band-selector switch I2 provided, for example, with two positions, one of which is for selectingv the short-wave frequency band and the other of which is for selecting the broadcast frequency band. Coupled to the radio-frequency ampliner il is an oscillator-modulator il having a similar two-position band-selector switch Il. Coupled in cascade to the oscillator-modulator Il are an intermediate-frequency amplifier I5 of any desired number of stages, a detector and A. V. C. supply IB which may include an audio-frequency amplifier tube, a tone-control system lll. an audio-frequency amplifier I8 of any desired number of stages, and a sound reproducer Il. The tone-control system Il is provided with a double-pole, double-throw switch 20 which is mechanically connected to the band-selector switches I2 and Il by means of a unicontrol 'mechanism U shown conventionally by broken lines. An automatic amplification control or A. V. C. bias derived from unit I0 is applied to the control grids of a suitable number of tubes in units I I, I3, and l5 over the conductor labeled A. V. C. It will be understood that the several parts of the system illustrated schematically may be conventional in their construction and operation; the details of which being well known in the art render a description thereof in detail unnecessary herein.

Neglecting for the moment the particular operation of the tone-control system I'I embodying the present invention, the above-described system includes the features of a conventional two-range superheterodyne receiver; the operation of such receiver being well known in the art, a detailed explanation thereof is unnecessary. Briefly, however, a desired modulatedcarrier signal intercepted by the ground-antenna circuit I0, I0 is selected and amplified in the radio-frequency amplifier II and converted by oscillator-modulator I3 to an intermediate-frequency signal. The intermediate-frequency signal is amplified by intermediate-frequency ampliiler I5 and translated therefrom to the detector i6, wherein the audio frequencies of modulation are derived and amplified. The audiofrequency signals are translated from unit I0 by the tone-control system I1 to the audiofrequency amplifier Il, wherein they are further amplified and delivered to sound reproducer i9 for reproduction in the usual manner. The unit I6 includes a conventional arrangement for deriving a unidirectional-bias voltage variable in accordance with the average amplitude of the received carrier wave, which is ap.. plied, as indicated, to the radio-frequency vamplifier Il, oscillator-modulator I3, and intermediate-frequency amplifier I5 automatically to control the amplification of these devices. to maintain the signal input to unit I0 within a relatively narrow range for a wide range of received signal intensities.

Referring now more particularly to the parts of the 'system comprising the present invention, the tone-control system I 1 comprises an input circuit including a vacuum tube Ila coupled to unit I0 and having a unite output impedance for supplying audio-frequency signals incidentally including noise which may extend over the entire audio-frequency range to a shunt r 2l which is coupled in the output circuitiof the tube ila. An output circuit including a resistor 22 having an adjustable contact 2l is coupled to the audio-frequency amplifier Il. A signaltrsnslating and -attenuating network interconnects the input and output circuits and. with resistors 2| and 22, comprises a pair of condensers 24 and 2i connected on one side to the signal-frequency high-potential terminal of the resistor 2| and, on the other side, to the signalfrequency high-potential terminal and to the adjustable contact 2i, respectively, of resistor 22 through a reversing switch 2l. The signalfrequency low-potential terminals of the resistors 2| and 22 are effectively connected together at signal frequencies by a conventional by-pass condenser 20. The values of the condenser 2l and the resistor 22` are proportioned to provide a high-pass signal-attenuating circuit having a time constant of not less than 0.0002 second and not greater than 0.0010 second so that this circuit is effective to attenuate by at least 6 decibels all signal and noise components of frequencies below a limiting frequency M118 between about and 500 cycles per second. The value of the second condenser 2l is so proportioned with respect to the value of the condenser 24 that, with the adjustable contact 28 adjacent the high-potential terminal of the resistor 22, for minimum tone-control action, the time constant of the circuit including the condensers 2l and 25 in parallel and the resistor 22, is not less than 0.0010 second. The values of the resistor 2i and the finite output impedance of the tube Ila are so proportioned with respect to the other circuit constants that, with the adjustable contact 23l adjacent the signal-frequency low-potential terminal of the resistor 22, for maximum tone-control action, all noise and sound-signal components above a limiting frequency lying be tween about 800 and 1500 cycles per second are attenuated by at least 6 decibels. With the adinstable contact 23 in its upper position, so as to include all of the resistance of the resistor 2! in circuit with the condensers 24 and 2l in parallel with each other, which is the position for normal operation with no tone-control action, the network passes substantially all noise and signal components at frequencies above a limiting lowfrequency value of not more than about 100 cycles with substantially no attenuation ofl the high-frequency components. Thus, the input and output circuits of the tone-control system are coupled by a signal-translating network including a plurality of signa1-attenuating portions, one of which is of the high-pass type and contains in series a resistor 22 and a condenser 2l and another of which is of low-pass type and contains in parallel a resistor 2i and a condenser 25 having a limiting frequency effective to attenuate by at least 6 decibels all signals above a limiting frequency between'800 and 1500 cycles, whereby the network effectively attenuates ail noise and sound signals at frequencies outside oi the middle range of frequencies that contribute most to intelligibility.

As an example of speciiic preferred values'of circuit constants which are effective to provide the foregoing characteristics, the vacuum tube may have a. nui-te output impedance of 100,000 ohms, the resistor 2l4 may have a resistance ot asses 0.25 megohm, the resistor 22 a resistance of 0.25

.megohm, the condenser 24 a capacitance of 0.0025 microfarad, and the condenser 25 a capacitance of 0.006 microfarad.

In considering the operation of the tone-control system I1, the unicontrol' mechanism U, which may be a push-button system, is moved to select a desired modulated-carrier signal-frequency band. Assuming that the unicontrol mechanism ismoved to selectA the short-wave band. the switches I2 and I4 are actuated to the positions illustrated by Fig. 1 of the drawing, while theswitch 20 is operated to its left-hand position. For normal operation of the tone-control system with minimum tone control, the contact 22 is adjusted to the upper terminal of resistor 22 so as to include substantially all of the resistance of the resistor 22 in series in a circuit which includes the condensers 24 and 25 in parallel with each other. as shown by Fig. 2 of the drawing. In this illustration the terminal 21 may represent an input terminal of the network, the terminal 28 may represent an output terminal of the network, and resistor 25 the plate resistance of tube Ila. With the network in this condition the tone-control system I1 has an output characteristic A, A as shown in Fig. 4. The condensers 24 and 25 are eifectively in parallel in a series circuit between th input and output terminals and, with the resistor 22, have a time constant of at least 0.0010 second, as pointed out above, and provide operation with minimum tone control. At half amplitude or 6 decibels attenuation the curve A, A has a lower cuto frequency of about 15-20 cycles. At progressively higher frequencies, the response increases gradually up to a predetermined frequency in a. conventional manner and is substantially uniformover the remainder `of the audio-frequency band utilized.

When the adjustable contact 23 is moved so as to exclude all of the resistance of the resistor 22 from the circuit including the condenser 25, the arrangement of the circuit elements is shown by Fig. 3 of the drawing. With the network in this condition, the time constant of the circuit.

including the condenser 24 and resistor 22 is not less than 0.0002 second and not greater than 0.0010 second which is effective to attenuate by at least 6 decibels all noise and signal components below a lower limiting frequency of not less'than 4 100 and not more than500 cycles. Also, the upof thel contact 23 is eective progressively to vary the time constants of the vnetwork between the values for which the network translates the useful audio-frequency components and the values for which the network eectively attenuates all noise and sound signals at frequencies outside of the middle range of frequencies that contribute most to intelligibility.

Characteristic curve B, B of Fig. 4 illustrates, by way of example, the response-frequency characteristic of the tone-control system I1 when the network is similar to that shown by, Fig. 3 with the .condensers 24 and 25 interchansed. which would be the condition when the unicontrol system U is moved to switch the receiver to broadcast band reception. For conventional broadcast band reception, however, the contact 23 of the resistor 22 preferably is adjusted to an intermediate position or to that in which the network is substantially in the condition shown by Fig. 2 of the drawing. With the latter adjustment, the response-frequency characteristic of the receiver is similar to that of curve A, Aof Fig. 4. As the contact 23 is adjusted progressively to narrow the band of audio-frequency signals translated by the tone-control system, the characteristics of the high-pass and low-pass attenuating portions being simultaneously adjusted until the curve A, A coincides with the curve B, B of Fig. 4. From these curves, it is seen that, for broadcast operation, the tone-control system is effective primarily to adjust the response of the receiver to high-frequency signals. Thus, the switchv 20 comprises means effective in one position lto interchange the condensers 24 and 25 for modifying the characteristics of the network to attenuate all noise and sound signals at frequencies outside of the middle range of frequencies that contribute most to intelligibility, the unicontrol connection U between the switch 20 and the bandselector switches I2 and I4 comprising unicontrol means for simultaneously operating the band selector of the receiver to select a high-frequency band andthe Itone-control means to the aforementioned position. Expressed in another way,

the switch 23 comprises means operable with the I The tone-control system' I1 is most effective t where severe noise conditions are encountered inv the reception of modulated-carrier signals in the short-wave frequency band. In this case, the adjustable contact 23 of the resistor 22 provides means for simultaneously adjusting the time constants of both the high-pass and low-pass signalattenuating portions of the circuit until the tonecontrol system Il effectively attenuates by at least 6 decibels all sound and noise at frequencies below the specined lower limiting frequency of between and 500 cycles, as well as all noise and sound at frequencies Iabove the specified upper limiting frequency of between 800 and 1500 cycles, translating only the middle range of fre'- quencies that contribute most to intelligibility. Speech within such range of frequencies is highly intelligible and substantially all of the noise is attenuated.

While there has been described what isat present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

v1. In an audio-frequency amplifier, a. tone-,control system comprising, an input circuit for supplying audio-frequency signals incidentally i'n-` 7 cluding noise which mayextend over the entire one of which is of high-pass type and contains v in series a resistor and a condenser having a time constant of not less than 0.0002 second and not greater than 0.0010 second and anotherof which is of low-.pass type and contains in parallela resistor and a condenser having a limiting frequency effective to attenuate by at least Q decibels all signals above a limiting frequency lying between 800 and 1500 cycles, whereby said network eifectively attenuates all noise and sound signals at frequencies outside of the middle range of frequencies that contribute most to intelligibility, and means for effectively progressively transferring said second condenser from said other signal-translating portion to said one signal-translating portion, simultaneously and progressively to change the time constants of both of said signal-attenuating portions, thereby progressively to change the frequency-band response of said network.

2. In an audio-frequency ampliner, a tone-control system comprising, an input circuit having finite impedance for supplying audio-frequency signals incidentally including noise which may extend over the entire audio-frequency range, an output circuit, and a signal-translating attenuating network coupled between said input and output circuits and including a first resistor connected across said input circuit, a second resistor connected across said output circuit, means effectively coupling .the signal-frequency low-potential terminals of said resistors, a first condenser coupling the signal-frequency high-potential terminal of said resistors, the values of said first condenser and second resistor being proportioned to provide an attenuating circuit having a time constant of not less than 0.0002 second and not greater than 0.0010 second, and a second condenser coupling the signal-frequency high-potential terminal of said first resistor and the signal-frequency low-potential terminal of said second resistor, the values of said first resistor and finite impedance being so proportioned with respect to the values of said condensers and said first resistor as to attenuate by at least 6 decibels all signals above a limiting frequency lying between 800 and 1500 cycles per second, whereby said network effectively attenuates all noise and sound signals at frequencies outside of the middle range of frequencies that contribute most to intelligibility.

3. In an audio-frequency amplier, a tonecontrol system comprising, an input `circuit including a vacuum tube having nite output impedance for supplying audio-frequency signals -incidentally including noise which may extend over the entire audio-frequency range, an output circuit, and a signal-translating attenuating network coupled between said input and output circuits and including a first resistor connected across said input circuit, a second resistor connected across said output circuit and having an adjustable contact, means effectively coupling the signal-frequency low-potential terminals of said resistors, a first condenser coupling the signal-frequency high-potential terminals of said resistors, the values of said first condenser and second resistor being proportioned to provide an attenuating portion having a time constant of not less than 0.0002 second and not greater than 0.0010 second, and a second condenser coupling the signal-frequency high-potential terminal of said first resistor and the adjustable contact of said second resistor for adjusting said network between maximum and minimum tone-control values, the values of said first resistor and nite impedance being so proportioned, with said adjustable contact adjacent the low-potential terminal of said second resistor for maximum tone assos control, as to attenuate by at least 6 decibels all signals above a limiting frequency lying between 800 and 1500 cycles Der second, whereby said network, with said contact adjusted for maximum tone control, effectively attenuates all noise and sound signals at frequencies outside of the mlddle range of frequencies that contribute most'to intelligibility.

4. In an audio-frequency amplifier,` a tonecontrol system comprising, an input circuit including a vacuum tube having a finite output impedance for supplying audio-frequency signals incidentally including noise which may extend over the entire audio-frequency range, an output circuit, and a signal-translating attenuating network coupled between said input and output circuits and including a first resistor connected across said input circuit, a second resistor connected across said output circuit and having an adjustable contact, means eifectively coupling the signal-frequency low-potential terminals of said resistors, a first condenser coupling the signalfrequency high-potential terminals of said resistors, the values of said first condenser and second resistor being proportioned to provide an attenuating circuit having a time constant of not less than 0.0002 second and not greater than 0.0010 second, and a second condenser coupling the signal-frequency high-potential terminal of said first resistor and the adjustable contact of said second resistor for adjusting said network between maximum and minimum tone-control values, the value of said second condenser being so proportioned with respect to the value of said first condenser that, with said adjustable contact adjacent the high-potential terminal of said second resistor for minimum tone control, the time constant of the circuit including said first and second condensers in parallel and said second resistor is not less than 0.0010 second, and the values of said first resistor and finite impedance being so proportioned with respect to the values of said condensers and said first resistor, with said adjustable contact adjacent the low-potential terminal of said second resistor for maximum tone control, as to attenuate by at least 6 decibels all signals above a limiting frequency lying between 800 and 1500 cycles, whereby said network, with said contact adjusted fox maximum tone control, effectively attenuates all noise and sound signals at frequencies outside of the middle range of frequencies that contribute most to intelligibility.

5. In an audio-frequency amplifier, a tonecontrol system comprising, an input circuit for supplying audio-frequency signals and incidentally including noise which may extend over the entire audio-frequency range, an output circuit, and a signal-translating attenuating network coupled between said input and output circuits and including a first resistor connected across said input circuit, a second resistor connected across said output circuit and having an adjustable contact, means effectively coupling the signal-frequency low-potential terminals of said resistors, a ilrst condenser coupling the signalfrequency high-potential terminals `of said resistors, the values of said ilrst condenser'and second resistor being proportioned to provide an attenuating circuit having a time constant of not les than 0.0002 second and not greater than 0.0010 second, andv a second condenser coupllng the signal-frequency high-potential terminal of said nrst resistor and the adjustable contact of said secondresistor for adjusting said network between maximum and minimum tonecontrol values, thevalue of said second condenser being so proportioned with respect to the 5 value of saidiirst condenser that, with said adjustable contact adjacent the high-potential terminal of said second resistor for minimum tone control, the time constant of the circuit including said first and second condensers in parallel l0 and said second resistor is not less than 0.0010 second, and the values of said first resistor and finite impedance being so proportioned with respect to the values of said condensers and said first resistor, with said adjustable contact adjacent the low-potential terminal of said second resistor for maximum tone control, as to attenuate by at least 6 decibels all signals above a limiting frequency lying between 800 and 1500 cycles, the adjustment of said contact being effective progressively to vary the time constants of said network between the values for which said network translates the useful audio-frequency components and the values for which said network effectively attenuates all noise and sound signals at frequencies outside of the middle range of frequencies that contribute most to intelligibility. 6. In a multiband modulated-carrier signal receiver having a band selector and an audio-frequency ampliier, a tone-control system comprising, an input circuit for supplyingl audio-frequency signals incidentally including noise which may extend over the entire audio-frequency range, an output circuit, and a signal-translating attenuating network coupled between said input and output circuits and including a plurality of signal-attenuating portions, one of which is of high-pass type and contains in series a resistor and a condenser having a time constant of between 0.0002 and 0.0010 lsecond and another of which is of low-pass type and contains in parallel a'resistor and a condenser having a limiting frequency of between 800 and 1500 cycles at half amplitude, tone-control means including means effective in one position to interchange said condensers for modifying the chai'- acteristics of said network to attenuate all noise and sound signals at frequencies outside of the middle range of frequencies that contribute most to intelligibility, and unicontrol means for simultaneously operating said band selector to select a high-frequency band and said tone-control means to said one position.

7. In a multiband modulated-carrier signal receiver having a band selector and an audiofrequency ampliiier, a tone-control system comprising, an input circuit for supplying audiofrequency signals incidentally including noise which may extend over the entire audio-frequency band, an output circuit, and a signaltranslating network coupled between said input and output circuits and including a plurality of signal-attenuating portions, one of which is of high-pass type and contains in series a resistor and a condenser having a time constant of not less than 0.0002v second and not greater than 0.0010 second and another of which is of lowpass type and contains in parallel a resistor and a condenser having a limiting frequency effective -to attenuate by at least 6 decibels all signals 5 above a limiting frequency lying between 800 and 1500 cycles, whereby said network effectively attenuates all noise and sound signals at fre- `quencies outside of the middle range of frequencies that contribute most to` intelligibility, 10 means for effectively progressively transferring said second condenser from said other signalattenuating portion to said one signal-attenuating portion simultaneously and progressively to t change the time constants of both of said signal- 15 attenuating portions, thereby progressively to increase the frequency band response of said network, and means operable with said band selector to select a predetermined band for interchanging said condensers to provide a different limiting- 20 frequency band-pass response characteristic for said network.

8. In a multiband modulated-carrier signal receiver having a band selector and an audiofrequency amplifier, a tone-control system com- 25 prising, an input circuit including a vacuum tube having finite output impedance for supplying audio-frequency signals incidentally including noise which may extend over the entire audiofrequency range, an output circuit, and a signal- 30 translating attenuating network coupled between said input and output circuitsy and including a first resistor connected across said input circuit, a second resistor connected across said output circuit and having an adjustable contact, means 35 effectively coupling thek signal-frequency lowpotential terminals of said resistors, a iirst condenser coupling the signal-frequency high-potential terminals of said resistors, the values of said first condenser and second resistor being 40 proportioned to provide an attenuating portion having a time constant of not less than 0.0002 second and not greater than 0.0010 second, aA

" second condenser coupling the signal-frequency network, with said contact adjusted for maximum tone control, effectively attenuates all noise and sound signals at frequencies outside of the middle range of frequencies that contribute most 00 to intelligibility, and means operable withsaid band selector toselect a predetermined band for interchanging said condensers.

, NELSON P. CASE. 35 

